Stem cell-based gene therapy offers a potential means to cure congenital severe hemoglobinopathies such as beta-thalassemia. For this reason we have constructed a lentiviral vector (TNS9) carrying the human beta-globin gene and demonstrated that with this vector we can obtain long-term correction of a mouse model affected by beta-thalassemia intermedia. Furthermore, this vector rescues a new lethal mouse model affected by beta-thalassemia major. However, in these mice the level of correction of the anemia and hemoglobin produced are not yet optimal. We believe that in order to unveil completely the potential of this gene therapy approach, we need to investigate, in this new mouse model of Cooley's anemia, (Aim 1) the correlation between the fraction of lentiviral transduced hematopoietic stem cells (HSC), the degree of BM chimerism and the corresponding level of anemia correction. For this purpose, we will generate a new lentiviral vector that combines expression of a reporter gene, such as the humanized red-shifted green fluorescent protein (hrGFP), in all the hematopoietic lineages and expression of the human beta-globin gene in erythroid cells (TNS9+GFP). To increase human beta-globin expression (Aim 2) we propose to generate new lentiviral vectors that could potentially increase hemoglobin production. We believe that we can raise hemoglobin production from TNS9 by extending the beta-globin promoter by 1 Kb and inserting a 1 Kb genomic region corresponding to the HS1 of the LCR. Another genomic element that could raise the level of hemoglobin production by diminishing the variability of expression at different genomic integration sites is the cHS4 insulator element. The production of new therapeutic vectors requires efficient strategies to compare and identify the best beta-globin encoding lentivirus. For this purpose we propose (Aim 3) to investigate the average level of expression of TNS9 versus the new lentiviral vectors (proposed in Aim 2) in BM chimeras. In addition, we propose (Aim 4) to generate transgenic mice from lentiviral transduced single copy ES cells to study the level of and variability in expression for each vector. Finally, insertion of selective drug resistance genes, dihydrofolate reductase (DHFR) versus methylguanine-DNA-methyltransferase (MGMT), will be evaluated (Aim 5) to enhance competitive repopulation of transduced stem cells expressing the human beta-globin gene.